1. Field of the Invention
The present invention concerns a pulling clamp for rapier looms of the type described in the introductory part of the claim 1 of the present invention and a preferred application of the inventive clamp.
2. Discussion of Background Information
In the textile industry rapier looms are in use often competing with other loom types (such as shuttle looms, projectile or air jet weaving machines, etc.) for inserting the weft threads into the weaving shed.
One of the major problems with these weaving machine types is the transfer of the weft thread at the centre of the fabric from the carrying clamp to the pulling clamp which transfer always is a susceptible phase in the weaving process particularly if synthetic fibres are processed composed of very delicate fibrils which are easily torn and in particular threads of glass fibrils which resist tensite forces but break very easily. The problems are aggravated if the threads are low twist or twistless threads which are applied in the production of technical fabrics of the highest quality in which any torn fibril sticking out of the fabric surface is considered a serious defect which can depreciate the product considerably.
Considerable efforts already have been taken in the past for solving the problem by providing a good grip of the thread in the pulling clamp of a rapier loom while ensuring at the same time a gentle treatment of the thread material and, as far as possible, excluding any rupture of fibrils.
A first design of a pulling clamp with an elastic tongue blocking the thread is described in CH-A-625573 in which document a pulling clamp is described comprising a hook and a gripping tongue moving at right angles with respect to the plane of the damp which furthermore is characterized in that the gripping action of the tongue is reinforced by an additional gripping element. This clamp thus is equipped with a gripping tongue which performs a motion of gripping transversal with respect to the clamp, i.e. the thread is gripped by the clamp from the side. As admitted by the applicant of said patent application a thread gripping system of such type cannot ensure sufficient safety as the tongue can oscillate laterally relative to the clamp itself during the fast longitudinal displacement of the clamp and thus can free the thread. For this reason the pulling clamp described according to the invention is provided with an additional gripping element which can eliminate the oscillations of the gripping tongue and thus can prevent the thread from escaping from the clamp. This system of "laterally" gripping the thread still did not prove sufficiently subtle for gripping threads composed of very breakable fibrils, particularly glass fibrils, and thus other solutions were studied for better gripping action which take care of the particular requirements in connection with the processing of technical fabrics produced from particularly delicate threads, even of types presenting high tensile strength, such as glass fibres.
An essential improvement was proposed in EP-A-0137377 which represents the state of the art relevant for the present invention. In the patent application cited a pulling clamp is proposed provided with a plunger guided and activated in the body of the clamp in such a manner that it alternatingly performs to and fro movements. On said plunger an elastic tongue is fastened which thus performs to and fro movements together with the plunger. The elastic tongue in this arrangement with its gripping zone is pressed against the gripping surface of the hook by the plunger when the plunger is located in its most forward position in such a manner that the thread is blocked between the tongue and the hook.
According to a further improvement of this solution, describeded in EP-A-0504899, for improved contact between the gripping zone of the tongue--fixed solidly to the plunger--and the gripping surface of the hook it was proposed that the plunger in its zone in which it contacts the tongue be provided with a cushion or runner made from an elastic material, e.g. from rubber, the function of which is to improve the adherence between the tongue and the hook of the clamp and thus to improve the grip on the thread.
The disadvantage of the state of the art described in the two last-mentioned documents cited, the first of which is considered as the state of the art pertinent to the present invention, is seen in that the elastic tongue, if present, is fastened to the plunger and thus performs the to and fro motion with the latter. This implies that the thread is gripped between the gripping zone of the elastic tongue and the gripping surface of the hook in a "dragging" movement due to the superimposition of the longitudinal to and fro movement of the plunger and the one at right angles to the gripping surface caused by the plunger which presses the tongue laterally against the gripping surface of the hook. This dragging movement of the tongue, however, is very disadvantageous as it inevitably causes a rolling movement of the thread about its axis: the fibrils thus are dragged along the walls of the elastic tongue and of the hook and are damaged as well as displaced from their position in the thread structure. An increase of ruptures of individual fibrils is the consequence of a dragging movement of such type and a kind of a false twist zone is generated in the thread: both phenomena are undesirable, particularly the second one if twistless filaments are processed.